The present invention relates to apparatus disposed at a first location and activated from a second location over a link connecting the first location and second location. In particular, the present invention relates to telephone remote isolation or disconnect devices. With more particularity, the present invention relates to a remote isolation device for disconnecting a customer device, for example, a telephone located remotely from a telephone central office, and which is activated by a control signal transmitted from the central office to the remote location. Remote isolation devices generally include a switching device which allows the customer equipment to be disconnected from the line in response to a signal transmitted over the telephone line. In some circuits of this type, the customer equipment is reconnected to the line after a time delay or is reconnected to the line in response to a further signal transmitted over the line. The present invention relates to such remote disconnect devices controlled by a direct current control signal and wherein the customer equipment is reconnected to the line a certain time delay after the d.c. disconnect control signal has been transmitted from the central office.
Apparatus connected to a telephone line and a customer's facility for the purpose of disconnecting customer equipment from the line and/or connecting further apparatus to the line in response to a signal sent over the telephone line from the telephone company office are generally known and are referred to as remote isolation or remote disconnect devices. The signal sent from the telephone central office can be d.c., pulsed or a.c., or combinations thereof.
An example of a remote isolation device which is actuated by a d.c. signal transmitted from the telephone central office is disclosed in commonly assigned U.S. Pat. Nos. 4,558,182 and 4,536,617. According to the circuits of these references, a d.c. signal is transmitted from the telephone company central office, the signal is polarity checked, subjected to a voltage protection circuit to protect the circuit from overloads, the d.c. signal is checked to determine if the voltage is above a fixed threshold, and if the voltage is above the threshold, the voltage is regulated to a prescribed voltage by a regulator. The output of the regulator is then supplied to a switching circuit which charges a capacitor during the time period that the voltage is applied to the telephone line. Once the voltage is removed from the telephone line, the capacitor stops charging, and, after a small time delay, the charge on the capacitor causes a transistor to conduct, energizing a relay. The charge on the capacitor, which can be reliably determined by the voltage regulator circuit, keeps the relay energized for a fixed time period. When the relay is energized, the subscriber equipment is disconnected from the line, and an impedance termination circuit, also known as a signature circuit, may be connected across the line. Accordingly, the telephone line can then be tested for the presence of the termination or signature circuit and test results after disconnection can be compared to pre-disconnection test results, thereby aiding in determination of fault location. For example, the central office can check to determine that the telephone line is not open or shorted if the signature circuit is sensed. If the signature circuit is not sensed by the central office, then it can be determined that a fault exists on the telephone line by virtue of the remote disconnect device not switching. If the signature circuit is sensed, it may be determined that a problem exists with the customer equipment or the customer house wiring, and not in the wiring from the central office leading into the customer premises, or alternatively it may be determined that a fault exists on the telephone line which still allows the disconnect device to switch, e.g., a high impedance short.
Since the charge on the capacitor in the device disclosed in the above two patents can be reliably determined, the amount of time during which the relay is energized, and thus the amount of time that the termination or signature circuit is connected across the telephone line is fixed. Once the capacitor discharges, the transistor which causes the relay to energize stops conducting, thus causing the relay to disconnect the signature circuit from the telephone line and reconnect the subscriber equipment.
Remote disconnect devices can also be used to disconnect customers on the occurrence of certain conditions, e.g., nonpayment.
Although the device disclosed in the above-identified patents provides an apparatus by which reliable time delays can be obtained during which the customer equipment used is connected and the signature circuit is connected across the lines, that device requires a large timing and charge storage capacitor. This is because the charge stored up while the d.c. signal is applied by the central office across the telephone line is used to energize the relay continuously until the charge decays and thus determines the time delay during which the relay is energized. The requirement for a large capacitor provides disadvantages in the circuit of the above two patents. For example, the large capacitor is expensive, physically bulky, less reliable than smaller capacitors, leakage is greater, component values are less stable, tolerances are greater, large time delays are not possible because the charge stored in the capacitor is used to energize the relay continuously, and because the capacitor is large, lengthy charging times are necessary. Furthermore, more energy must be transmitted from the central office, thus making designs using large charging capacitors less efficient and making them less useful in situations where severe faults are present on the line.
An alternative circuit in the above two patents utilizes latching relays and smaller charging capacitors. That circuit (see FIG. 5 of U.S. Pat. No. 4,536,617), however, utilizes a first signal of one polarity to disconnect the subscriber equipment and a second signal of another polarity to reconnect it.